1. Field of the Invention
The present invention relates to an image forming device for forming an image on a roll of photosensitive/pressure-sensitive recording medium.
2. Description of the Related Art
There has been known an image forming device for forming an image on a roll of photosensitive/pressure-sensitive recording medium that includes microcapsules with dye precursor. In this kind of image forming device, the roll of recording medium is cut down to a predetermined output size by a cutter, which is located at a cutting position. Then, an exposure unit exposes the cut recording medium to a light of a certain wavelength. The microcapsules selectively harden by reacting to the light, and a latent image is formed in the recording medium accordingly. Then, a pair of pressing rollers of a developing unit apply pressure to the recording medium sandwiched therebetween. As a result, unhardened microcapsules are ruptured, and dye precursor exudes from the ruptured microcapsules, thereby developing an image corresponding to the latent image. Afterwards, the image is thermally fixed by a fixing unit.
However, when the recording medium is cut down to the output size, adhesive materials included in the recording medium exude from the cut surface and adhere onto the cutter. The adhesive materials will gradually accumulate on the cutter until eventually the cutter becomes unable to cut the recording medium.
Also, mechanical stress is applied to the recording medium during the cutting operation. This ruptures the microcapsules around the portion of the recording medium, so that the cutting portion of the cut recording medium may be developed in an undesirable color, thereby degrading quality of the developed image.
In order to overcome this problem, there has been proposed an image forming device including the cutting unit shown in FIG. 1. In this image forming device, an exposure unit 850 exposes a cut portion of a recording medium 810 with white light from the above. Then the recording medium 810 is cut along the exposed cut portion by the cutter 840. More specifically, as shown in FIG. 1, a linear light source 820 emits white light. The light source 820 extends to a greater length than the width of the recording medium 810. The light emitted from the light source 820 reaches the recording medium 810 through a slit 830 having a predetermined width. Because the cutter 840 cannot be positioned directly below the light source 820, the cutter 840 is located at a position remote from the exposure unit 840. Therefore, the recording medium 810 is transported to a cutting position after the exposure operation. The recording medium 810 is placed between a pair of blades 840a, 840b, and cut along the exposed cutting portion. Because the microcapsules at the exposed cutting portion of the recording medium 810 are all hardened, the microcapsules will not be undesirably ruptured at the cutting operation. Therefore, a high quality image can be provided.
However, the recording medium 810 may be inaccurately transported from the through hole to the cutting position because the feeding mechanism slips or for some other reason. If the cutting portion is exposed to only a narrow width, then when the recording medium 810 is inaccurately transported, it may be cut in front or behind the exposed cutting portion. Therefore, the exposed cutting portion must be formed to have a certain wide width. However, the an image cannot be formed on the exposed cutting portion, so the recording medium 810 is wasted when the cutting portion is formed wide.
Also, because the recording medium 810 is first exposed with a light, transported to the cutting position, and then cut by the cutter 840, the overall operation takes a relatively long time. Also, because the exposure unit 850 and the cutter 840 are positioned separated from each other, the image forming device has a relatively large size.
Moreover, because a recording medium has a certain thickness, a large shock is applied to the developing unit when the recording medium is first inserted in between the pressing rollers and later discharged from between the pressing rollers. This large shock can produce a loud noise, and can also affect the developing unit, thereby reducing the life of the developing unit.
In order to overcome this problem, Japanese Patent-Application Publication No. HEI-1-300256 discloses a mechanism for cutting down a recording medium at a predetermined angle with respect to a feed direction of the recording medium. That is to say, the recording medium is cut perpendicular to a surface of the recording medium, but at a slant with respect to a widthwise direction of the recording medium. When the recording medium cut in this manner is inserted in between the pressing rollers, a leading edge of the recording medium is gradually inserted in between the pressing rollers, so that the shock is less than in the situation described above. The same is the case when the rear edge of the recording medium is discharged from between the pressing rollers.
However, in this case, when the recording medium is cut at a slant in this manner, the surface area of the portion sandwiched between the pressing rollers will change gradually when the edge portions enter or leave the pressing rollers. Therefore, the pressure per unit surface area on the leading and rear edge portions of the recording medium changes in association with distance that the recording medium is transported. Microcapsules are ruptured in varying amounts depending on pressure applied, so that the color of the developed image will be uneven.
Also, the amount of compression energy that accumulates on the rear edge of the recording medium is much greater than at positions where the width is wider. As a result, the recording medium can fly out of the developing unit with a popping or other an unusual sound. Therefore, pressing development can be sometimes insufficient.